The manufacturing of IC devices involves the sequential processing of a semiconductor wafer to add or remove various layers of materials. The critical layers define features in the IC devices that possess tighter groundrules, such as those for isolation, gate, contact, and first metal, while the non-critical layers define the remaining features.
The process of photolithography is performed to pattern the layers on the wafer. Initially, the wafer is covered with photoresist or other material that is sensitive to light energy. Then, an imaging tool, such as a stepper, aligns the wafer and a mask prior to projecting light energy onto the mask. The light energy is subsequently transmitted or reflected to print a latent image in the photoresist on the wafer. After printing the same image in contiguous fields across the wafer, the photoresist on the wafer is developed to create openings that correspond to the placement and intensity of the light energy. The image transfer to the wafer may further involve the processes of etch or ion implantation. For example, the photoresist may act as a stencil for transferring a pattern from the mask into a topographical structure or a dopant profile.
Contaminants must be kept away from the vicinity of a mask during exposure in order to ensure fidelity of the pattern transfer. The features on a mask may be kept clean by covering with a pellicle. The pellicle is a thin, yet robust, layer of a material that is transparent to the exposure light energy.
A shorter wavelength may be selected for the exposure light in order to print a smaller critical dimension (CD). However, very few materials are sufficiently transparent or durable when exposed to light energy having a wavelength of less than about 180 nm.
Thus, what is needed is an apparatus for and a method of keeping contaminants away from the vicinity of a mask during exposure.